﻿using SharpZLibRT.Checksums;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace SharpZLibRT
{
    public class GZipInputStream : InflaterInputStream
    {
        #region Instance Fields
        /// <summary>
        /// CRC-32 value for uncompressed data
        /// </summary>
        protected Crc32 crc;

        /// <summary>
        /// Flag to indicate if we've read the GZIP header yet for the current member (block of compressed data).
        /// This is tracked per-block as the file is parsed.
        /// </summary>
        bool readGZIPHeader;
        #endregion

        #region Constructors
        /// <summary>
        /// Creates a GZipInputStream with the default buffer size
        /// </summary>
        /// <param name="baseInputStream">
        /// The stream to read compressed data from (baseInputStream GZIP format)
        /// </param>
        public GZipInputStream(Stream baseInputStream)
            : this(baseInputStream, 4096)
        {
        }

        /// <summary>
        /// Creates a GZIPInputStream with the specified buffer size
        /// </summary>
        /// <param name="baseInputStream">
        /// The stream to read compressed data from (baseInputStream GZIP format)
        /// </param>
        /// <param name="size">
        /// Size of the buffer to use
        /// </param>
        public GZipInputStream(Stream baseInputStream, int size)
            : base(baseInputStream, new Inflater(true), size)
        {
        }
        #endregion

        #region Stream overrides
        /// <summary>
        /// Reads uncompressed data into an array of bytes
        /// </summary>
        /// <param name="buffer">
        /// The buffer to read uncompressed data into
        /// </param>
        /// <param name="offset">
        /// The offset indicating where the data should be placed
        /// </param>
        /// <param name="count">
        /// The number of uncompressed bytes to be read
        /// </param>
        /// <returns>Returns the number of bytes actually read.</returns>
        public override int Read(byte[] buffer, int offset, int count)
        {
            // A GZIP file can contain multiple blocks of compressed data, although this is quite rare.
            // A compressed block could potentially be empty, so we need to loop until we reach EOF or
            // we find data.
            while (true)
            {

                // If we haven't read the header for this block, read it
                if (!readGZIPHeader)
                {

                    // Try to read header. If there is no header (0 bytes available), this is EOF. If there is
                    // an incomplete header, this will throw an exception.
                    if (!ReadHeader())
                    {
                        return 0;
                    }
                }

                // Try to read compressed data
                int bytesRead = base.Read(buffer, offset, count);
                if (bytesRead > 0)
                {
                    crc.Update(buffer, offset, bytesRead);
                }

                // If this is the end of stream, read the footer
                if (inf.IsFinished)
                {
                    ReadFooter();
                }

                if (bytesRead > 0)
                {
                    return bytesRead;
                }
            }
        }
        #endregion

        #region Support routines
        bool ReadHeader()
        {
            // Initialize CRC for this block
            crc = new Crc32();

            // Make sure there is data in file. We can't rely on ReadLeByte() to fill the buffer, as this could be EOF,
            // which is fine, but ReadLeByte() throws an exception if it doesn't find data, so we do this part ourselves.
            if (inputBuffer.Available <= 0)
            {
                inputBuffer.Fill();
                if (inputBuffer.Available <= 0)
                {
                    // No header, EOF.
                    return false;
                }
            }

            // 1. Check the two magic bytes
            Crc32 headCRC = new Crc32();
            int magic = inputBuffer.ReadLeByte();

            if (magic < 0)
            {
                throw new EndOfStreamException("EOS reading GZIP header");
            }

            headCRC.Update(magic);
            if (magic != (GZipConstants.GZIP_MAGIC >> 8))
            {
                throw new Exception("Error GZIP header, first magic byte doesn't match");
            }

            //magic = baseInputStream.ReadByte();
            magic = inputBuffer.ReadLeByte();

            if (magic < 0)
            {
                throw new EndOfStreamException("EOS reading GZIP header");
            }

            if (magic != (GZipConstants.GZIP_MAGIC & 0xFF))
            {
                throw new Exception("Error GZIP header,  second magic byte doesn't match");
            }

            headCRC.Update(magic);

            // 2. Check the compression type (must be 8)
            int compressionType = inputBuffer.ReadLeByte();

            if (compressionType < 0)
            {
                throw new EndOfStreamException("EOS reading GZIP header");
            }

            if (compressionType != 8)
            {
                throw new Exception("Error GZIP header, data not in deflate format");
            }
            headCRC.Update(compressionType);

            // 3. Check the flags
            int flags = inputBuffer.ReadLeByte();
            if (flags < 0)
            {
                throw new EndOfStreamException("EOS reading GZIP header");
            }
            headCRC.Update(flags);

            /*    This flag byte is divided into individual bits as follows:

            bit 0   FTEXT
            bit 1   FHCRC
            bit 2   FEXTRA
            bit 3   FNAME
            bit 4   FCOMMENT
            bit 5   reserved
            bit 6   reserved
            bit 7   reserved
            */

            // 3.1 Check the reserved bits are zero

            if ((flags & 0xE0) != 0)
            {
                throw new Exception("Reserved flag bits in GZIP header != 0");
            }

            // 4.-6. Skip the modification time, extra flags, and OS type
            for (int i = 0; i < 6; i++)
            {
                int readByte = inputBuffer.ReadLeByte();
                if (readByte < 0)
                {
                    throw new EndOfStreamException("EOS reading GZIP header");
                }
                headCRC.Update(readByte);
            }

            // 7. Read extra field
            if ((flags & GZipConstants.FEXTRA) != 0)
            {
                // Skip subfield id
                for (int i = 0; i < 2; i++)
                {
                    int readByte = inputBuffer.ReadLeByte();
                    if (readByte < 0)
                    {
                        throw new EndOfStreamException("EOS reading GZIP header");
                    }
                    headCRC.Update(readByte);
                }

                if (inputBuffer.ReadLeByte() < 0 || inputBuffer.ReadLeByte() < 0)
                {
                    throw new EndOfStreamException("EOS reading GZIP header");
                }

                int len1, len2;
                len1 = inputBuffer.ReadLeByte();
                len2 = inputBuffer.ReadLeByte();
                if ((len1 < 0) || (len2 < 0))
                {
                    throw new EndOfStreamException("EOS reading GZIP header");
                }
                headCRC.Update(len1);
                headCRC.Update(len2);

                int extraLen = (len1 << 8) | len2;
                for (int i = 0; i < extraLen; i++)
                {
                    int readByte = inputBuffer.ReadLeByte();
                    if (readByte < 0)
                    {
                        throw new EndOfStreamException("EOS reading GZIP header");
                    }
                    headCRC.Update(readByte);
                }
            }

            // 8. Read file name
            if ((flags & GZipConstants.FNAME) != 0)
            {
                int readByte;
                while ((readByte = inputBuffer.ReadLeByte()) > 0)
                {
                    headCRC.Update(readByte);
                }

                if (readByte < 0)
                {
                    throw new EndOfStreamException("EOS reading GZIP header");
                }
                headCRC.Update(readByte);
            }

            // 9. Read comment
            if ((flags & GZipConstants.FCOMMENT) != 0)
            {
                int readByte;
                while ((readByte = inputBuffer.ReadLeByte()) > 0)
                {
                    headCRC.Update(readByte);
                }

                if (readByte < 0)
                {
                    throw new EndOfStreamException("EOS reading GZIP header");
                }

                headCRC.Update(readByte);
            }

            // 10. Read header CRC
            if ((flags & GZipConstants.FHCRC) != 0)
            {
                int tempByte;
                int crcval = inputBuffer.ReadLeByte();
                if (crcval < 0)
                {
                    throw new EndOfStreamException("EOS reading GZIP header");
                }

                tempByte = inputBuffer.ReadLeByte();
                if (tempByte < 0)
                {
                    throw new EndOfStreamException("EOS reading GZIP header");
                }

                crcval = (crcval << 8) | tempByte;
                if (crcval != ((int)headCRC.Value & 0xffff))
                {
                    throw new Exception("Header CRC value mismatch");
                }
            }

            readGZIPHeader = true;
            return true;
        }

        void ReadFooter()
        {
            byte[] footer = new byte[8];

            // End of stream; reclaim all bytes from inf, read the final byte count, and reset the inflator
            long bytesRead = inf.TotalOut & 0xffffffff;
            inputBuffer.Available += inf.RemainingInput;
            inf.Reset();

            // Read footer from inputBuffer
            int needed = 8;
            while (needed > 0)
            {
                int count = inputBuffer.ReadClearTextBuffer(footer, 8 - needed, needed);
                if (count <= 0)
                {
                    throw new EndOfStreamException("EOS reading GZIP footer");
                }
                needed -= count; // Jewel Jan 16
            }

            // Calculate CRC
            int crcval = (footer[0] & 0xff) | ((footer[1] & 0xff) << 8) | ((footer[2] & 0xff) << 16) | (footer[3] << 24);
            if (crcval != (int)crc.Value)
            {
                throw new Exception("GZIP crc sum mismatch, theirs \"" + crcval + "\" and ours \"" + (int)crc.Value);
            }

            // NOTE The total here is the original total modulo 2 ^ 32.
            uint total =
                (uint)((uint)footer[4] & 0xff) |
                (uint)(((uint)footer[5] & 0xff) << 8) |
                (uint)(((uint)footer[6] & 0xff) << 16) |
                (uint)((uint)footer[7] << 24);

            if (bytesRead != total)
            {
                throw new Exception("Number of bytes mismatch in footer");
            }

            // Mark header read as false so if another header exists, we'll continue reading through the file
            readGZIPHeader = false;
        }
        #endregion
    }
}
